Hypothesis 3:

Neoplasia is a consideration in this causing Bonnie's lameness.  Pathophysiology will go here.

We believe that osteosarcoma could be the cause of this lameness because of Bonnie's signalment and clinical signs.  Bonnie is six years old and is within the age range of peak incidence for this disease, which is between the ages of five and nine years.  85% of all canine bone tumors are osteosarcomas.  Osteosarcomas are prevalent in Labrador retrievers, giving her a high breed incidence.  The location of Bonnie's lameness appears to be the stifle joint, and a prevalent site of osteosarcoma is the proximal tibia.  Bonnie's sudden onset of pain and lameness with history of trauma could be explained by the presence of a pathologic fracture.  Many times a pathologic fracture is the first clinical sign seen in a dog with osteosarcoma.  The pain caused by this fracture could lead to Bonnie's partial weight bearing lameness and to the pain on manipulation of the stifle. 
Dogs with early osteosarcoma are initially in good health as seen with Bonnie.  An aggressive tumor such as an osteosarcoma will present with focal soft tissue swelling, as is the case with Bonnie's stifle.  Bonnie presented with effusion of the left stifle joint with distension of the joint capsule.
Osteosarcoma would be easily diagnosed on radiographs.  Osteosarcomas appear on radiographs as an aggressive bone tumor with focal soft tissue swelling.  The metaphysis is the most common location for this neoplasm, but it can be located in other parts of the bone.  There would be evidence of cortical and medullary bone lysis and disorganized new bone formation that invades the surrounding tissues.  In 50% of the cases a sunburst appearance occurs as periosteal new bone radiates out from the tumor.  A pathologic fracture could be visible.  It is also possible that joint involvement would be seen.  When suspecting osteosarcoma, it is important to also radiograph the lungs to check for metastasis.  The tumor foci would appear as rounded discrete opacities in the lungs.  If radiographic evidence points to osteosarcoma, further investigation of the disease can be accomplished through a bone biopsy.